Yeah, I love when kits come with PE instrument panels. Meanwhile, the Yahu pre-made instrument panels are incredible, but I don't think they have a P-61 panel.
One big difference between the DML P-61A kit and the A&A F-15 kit is the main gear bays.
The P-61A booms are solid at the top (you don't see the lower surface of the wing through them) and the only detail they have is on the sides. There are no bulkheads, either.
The F-15 booms are open at the top, so you do see the lower surface of the wing and the aft spar and they have both forward and aft bulkheads.
So if you are going to pose the main gear doors open, there's a fair amount of detail for 1/72 scale.
However, the F-15 (like that P-61B and later Black Widows) had the cyclic doors that mostly close after the landing gear is extended. Most photos of the F-15 show the main gear doors closed on the ground, except for the small outer door at the front (like a B-25). They probably opened the main doors for maintenance, however, and I've seen one photo that does show the doors fully open.
Since my P-61A model will have the doors fully open (as it should), I am swapping wings and booms between it and the F-15 model since it offers more detail. I am keeping the F-15 doors closed, but you could certainly model it either way if you wanted.
These photos show how it all fits together. I would suggest assembling the wings and then the booms without the forward bulkhead. The forward bulkhead should be added later because it affects the alignment of the boom to the wing. If you don't glue it in exactly the right spot, the boom and wing will not mate properly.
So build the wing, build the boom, then dry fit the bulkhead and attach them wing and boom. Come to think of it, you really don't even have to glue the bulkhead in because it will be trapped in place. Maybe just add a drop of glue to it after installation.
One nice thing DML did was to keep one end of the main landing gear sockets open so that you can attach the main gear legs AFTER assembling the boom. The Monogram P-61 required you to attach the gear leg when gluing the boom halves together. I think I am going to make this same mod to my Monogram kit, too.